Double-coiled tungsten filament lamps as absolute spectral irradiance reference sources

We have developed a physical model for the spectral behaviour of the radiation of tungsten filament lamps. The model is based on Planck's radiation law, published values for the emissivity of tungsten, measurement geometry and measured values for a residual correction function. In this paper, we study the physical interpretation of the model. The studied properties influencing the lamp spectra include the emissivity data used, light recycling effect due to the cylindrical shape of the filament, transmittance of the glass envelope and the halogen gas and temperature dependence of the transmittance. It appears that in addition to tungsten emissivity, one needs to account for the bulb transmittance and the light recycling within the cylindrical filament. We did not observe any wavelength dependence of the absorption of the filling gas.